The present invention relates generally to robotic machine tools, and, more specifically, to calibration thereof.
Robotic machine tools exist in various configurations for performing various manufacturing operations in the production of various machine components. A typical machine tool is supported in a multi-axis machine for following a programmed path over the contours of a three dimensional workpiece. The workpiece may require precise machining of its surface contour to a specific shape, or may require welding at specific locations on the contour, or may require coating of its surface, for example.
In one exemplary configuration, a plasma torch or gun is mounted to the distal end of an articulated robotic arm having multiple degrees of movement such as translation or rotation or both. The machine may be programmed to aim the plasma gun toward the surface of the workpiece and follow a programmed path for automatically plasma spraying the workpiece with a suitable material.
For example, the workpiece may be a gas turbine engine stator vane having a complex 3-D contour requiring the deposition of a thermal barrier coating thereon by plasma spraying. In order to plasma spray a uniform coating over the entire surface of the workpiece, the plasma gun must follow a precise spraying path while maintaining a suitable offset or standoff from the surface of the workpiece.
The path program may be learned by the machine by manually repositioning the plasma gun from point-to-point around the surface of the workpiece while recording the corresponding position coordinates of the gun at each desired point in the path. The path is typically a rectangular grid of many discrete points at which the plasma gun may be aimed during the programmed sequence. Alternatively, the programmed path for the plasma gun may be analytically determined using the motion capabilities of the machine relative to the supported workpiece therein.
In either case, the machine must be suitably calibrated for ensuring accuracy of the programmed path relative to an individual workpiece. Even nominally identical workpieces are subject to manufacturing tolerances which affect the final dimensions thereof. Accordingly, it is typically desired to calibrate the machine for each part being handled to maximize the accuracy of the manufacturing process being applied thereto.
Robotic machine calibration is typically effected by using a rubber or spring contact pointer specifically configured for mounting to the robotic arm which permits the arm to follow the desired path so that the pointer may contact the workpiece without damage thereto. The pointer requires careful movement of the robotic arm as the pointer approaches the workpiece and withdraws therefrom to avoid contacting any adjacent portions of the workpiece which might be damaged thereby. This process is deliberate and slow and subject to inaccuracy as the pointer is compressed at its intended contact points. And, the contact between the pointer and workpiece may contaminate the workpiece.
For example, plasma spraying requires a clean workpiece which would be contaminated by pointer contact therewith. Calibration of the programmed path using the contact pointer necessarily requires an additional processing step to clean the workpiece after calibration and prior to plasma spraying.
Contact pointers thusly limit the speed at which at which robot program paths may be developed. They also limit the accuracy of machine tool position during the programmed path. They may also contaminate the workpiece on contact therewith, and may cause damage to the workpiece itself or the robotic machine when they inadvertently contact the workpiece during travel.
Accordingly, it is desired to provide a non-contact calibration pointer for a multi-axis machine tool for improving the calibration and manufacturing process.
A robotic machine includes a machine tool removably supported in a mount for following a programmable path over a workpiece. A calibration pointer includes a housing configured like the tool for being supported in the mount, and a laser is affixed in the housing for emitting a laser beam at the workpiece. In a method of operation, the laser beam is projected from the laser in a focused spot at the workpiece at an offset therefrom. The spot permits accurate programming of the machine without contacting the workpiece.